The present invention relates to a device for directly reading from a chart the heading to be followed by an air or sea-going vehicle and the estimated time which this vehicle should take to make a journey between two geographical references, this information being obtained without having to calculate a drift correction and magnetic declination.
It should first of all be noted that the "estimated time" mentioned above is the time actually taken by the vehicle, taking into account phenomena such as wind or sea currents, as opposed to the time without wind or without current.
Furthermore, the "ground course" in air navigation or "bottom course" in sea navigation, is the course covered with respect to the ground (to the sea bottom) and, consequently, on the chart. It is therefore the course which is fixed as objective.
The presence of the wind (or of the current) requires, for following the same course to be covered, a drift correction and a fictitious heading called "true heading", which is the angle formed by the axis of the vehicle with the geographical North. It will be recalled in this connection that the "magnetic heading" is the heading with respect to the magnetic North; it is derived from the "true heading" by addition of the magnetic declination.
The "true heading" is calculated by a vectorial composition of the "wind speed", "vehicle speed" vectors (detected with respect to air or the water), and the "ground speed" or "bottom speed"; this composition is usually called "speed triangle".
These vectors are of sliding types, i.e. their point of application is movable and is therefore not defined.
In an orthonormed system, these vectors are then defined by their arguments (angle formed by a reference direction and the direction of the vector) and their modulus.
Thus:
the "wind speed" vector will have as argument the direction of the wind and, as modulus, the absolute value of the wind speed, these two pieces of information forming data, PA1 the "vehicle speed" vector will have as argument the "true heading" of the vehicle which is unknown and, as modulus, the absolute value of the speed of the vehicle with respect to air (or to the water) which forms a data, PA1 the "ground speed" vector will have as argument the "ground (or bottom) course" which is a data and, as modulus, a value forming an unknown. PA1 determination of the wind speed/proper speed ratio, for example by means of an abacus; PA1 adjustment of the off-centering of the disk according to the value of the previously obtained ratio, the orientation of this offset being determined by setting the first reference mark on a value of the circular graduation corresponding to the orientation of the wind (meteorological data); PA1 the setting of the zero of the circular graduation on the North; and PA1 reading the true heading which is indicated on the circular graduation by a straight line oriented along the geographical heading and passing through said axis.
This vectorial system comprises then four data and two unknowns.